The coastal plant near San Clemente once supplied power to about 1.4 million homes in Southern California but has been shuttered since January 2012 when a tube in its newly replaced steam generators leaked a small amount of radioactive steam, leading to the discovery that the tubes were wearing down at an unusual rate.

San Onofre, one of only two nuclear power plants in California, has been in regulatory limbo for months as the U.S. Nuclear Regulatory Commission weighed a proposal by Southern California Edison to restart one unit -- which was less heavily damaged -- and run it at 70% power in hopes that reduced power would alleviate the conditions that led to the wear.

"I am greatly relieved that the San Onofre nuclear plant will be closed permanently," Boxer said in a statement. "This nuclear plant had a defective redesign and could no longer operate as intended."

Boxer has called on the U.S. Department of Justice to investigate whether Edison misled regulators about the extent of design changes in the new generators. The NRC's office of investigations and office of inspector general were already separately probing whether there was any wrongdoing.

Shaun Burnie, a nuclear campaigner with the environmental group Friends of the Earth, also hailed the decision.

"It has great national implications and is a real strong message that this nation does not need nuclear power," he said.

Friends of the Earth has filed a series of motions with the NRC in an attempt to slow or stop the restart of the plant. A recent ruling by a special NRC panel of judges in response to a Friends of the Earth petition found that the restart process was a de facto license amendment that would require an opportunity for a lengthy public hearing process.

Donna Gilmore, a San Clemente resident who runs a blog focused on safety issues at the plant, said, "I'm in disbelief that this has finally happened. I expected it to be a much longer battle."

But Gilmore said she still has concerns about the waste that will be stored at San Onofre after the shutdown.

Southern California Edison officials announced their decision in a news release early Friday morning.

"We have concluded that the continuing uncertainty about when or if [the plant] might return to service was not good for our customers, our investors, or the need to plan for our region’s long-term electricity needs," said Ted Craver, chairman and chief executive of Edison International, parent company of SCE.

SCE President Ron Litzinger said in a statement: "Looking ahead, we think that our decision to retire the units will eliminate uncertainty and facilitate orderly planning for California’s energy future.”

The shutdown of the plant will mean a reduction in staff at the plant from about 1,500 to 400.

Dan Dominguez, business manager for the Utility Workers Union of America Local 246, said he had gotten word of Edison's decision early Friday morning. Although Edison had hinted that the plant might be retired, Dominguez said the news still came as a shock.

Dominguez said he was disappointed and thought that the plant could have run safely if allowed to restart.

"But the decision's been made, so we'll have to deal with the consequences," he said.